The influence of particle size (d_P) and solid concentration (C_s) on the hydrodynamics of a slurry bubble column were studied experimentally using ultrafast electron beam X-ray tomography. Experiments were carried out for d_P=50-150μm at C_s=0-0.20 and superficial gas velocity U_G=0.02-0.05m/s in a slurry bubble column of diameter D_T=0.07m. The real time 3D gas flow structure, average bubble diameter and averaged gas hold-up were found to be independent of d_P and C_s at small d_P≤100μm and low C_s≤0.03, however, at large d_P and high C_s, strong dependency on d_P and C_s were observed. Corresponding three-phase simulations were also performed using the Euler-Euler multi-fluid model implemented in CFX 14. The bubble size distribution was calculated using the MUSIG model. The predicted gas hold-up and bubble size distribution also shows dependence on d_P and agreed quite well with the measurements and previous studies.